1. Field of the Invention
The present invention is generally related to the field of pressure control devices, and, in one particular example, to a pressure balanced fluid control device.
2. Description of the Related Art
Fluid control devices, such as gate valves, are commonly employed in a variety of different industries, e.g., surface and subsea oil and gas drilling and producing industries. Gate valves may be classified as either balanced stem or unbalanced stem valves. As is well known in the art, in an unbalanced stem gate valve, there is a net force exerted on the valve stem. In some cases, this force may be equal to the pressure exerted within the valve body multiplied by the seal area of the stem. In unbalanced gate valve designs, it is often necessary to limit the stem seal area to reduce the unbalanced stem force and thus the operating torque required to actuate the gate valve. Although reducing the stem seal area may reduce unbalanced forces, such a limitation may place a number of undesirable constraints on the design of a gate valve.
In some embodiments, particularly surface valves, balanced stem gate valves include a second stem attached to the opposite end of the gate wherein the second stem has essentially the same seal area as the valve stem. Accordingly, there is no net force acting on the valve stem. One drawback to the use of such a balanced stem design is that the second stem increases the size, cost and complexity of the valve, and introduces additional potential leak paths. For example, in many subsea applications, there is no available space for incorporating a balance stem into the design of the subsea equipment, e.g., a Christmas tree.
FIG. 1 depicts one illustrative embodiment of a prior art balanced stem gate valve 10. In general, the gate valve 10 is comprised of a body 12, a bonnet 14, a gate 16 having an opening 18 formed therethrough, and a seal 15, e.g., typically a metal gasket. The gate valve 10 further comprises a plurality of flanges 29 and it has an opening 31 formed therethrough to allow working fluid to pass through the gate valve 10 when the gate 16 is positioned such that the opening 18 in the gate 16 is aligned with the opening 31 through the body 12. The bonnet 14 is secured to the body 12 by a plurality of nuts and studs, generally indicated by the reference number 14a. The gate valve 10 further includes a valve stem 20 and a pressure balance stem 22. The valve stem 20 is coupled to the gate 16 by a lifting nut 21. A pin 21A couples the lifting nut 21 to the stem 20 such that there is no rotation between these components. A directional valve stem seal 11, e.g., UV elastomer packing, is positioned in the bonnet 14 around the valve stem 20. The pressure balance stem 22 is positioned within a balance stem housing 22a that is coupled to the body 12 by a plurality of studs and nuts, generally indicated by the reference number 22b. A directional seal 13 is positioned around the pressure balance stem 22.
The gate valve 10 further comprises means for actuating the gate 16. In the depicted embodiment, the valve 10 employs a roller screw assembly 24 and an actuator handle assembly 28. Hydraulic or electrical actuators can also be employed with the valve 10 if desired. The roller screw assembly 24 is partially positioned within a bonnet cap 26 that is threadingly coupled to the bonnet 14 at a threaded connection 27. A plurality of set screws 37 are used to further secure the bonnet cap 26 to the bonnet 14 after the threaded connection 27 is made. The roller screw assembly 24 further comprises a roller screw 30, a threaded nut 32, a roller screw housing 25, a plurality of bearings 36 and a key 34. The key 34 couples the roller screw nut 32 to the roller screw housing 25. An elastomer seal 40 is provided between the bonnet cap 26 and the roller screw housing 25. An elastomer seal 41 is positioned between the bonnet 14 and the bonnet cap 26. Also depicted in FIG. 1 are a plurality of openings 42 having threaded fasteners 43, e.g., screws, positioned therein. The openings 42 are provided as a means of providing lubricant to the bearings 36. In some cases, e.g., sub-surface applications, such openings would not typically be present. The valve stem 20 is threadingly coupled to the roller screw 30 via a threaded connection 33 (external threads on an end of the valve stem 20 engage an internally threaded opening formed in the roller screw 30). The valve stem 20 may further be coupled to the roller screw 30 by a pin 23.
Historically, gate valves have required a relatively large amount of torque to actuate the valve, e.g., actuation systems involving the engagement of high friction ACME threads. However, such high torque actuation systems present several problems. For example, with respect to manually actuated valves, such high torque valves required the generation of relatively large forces, thus making actuating such gate valves difficult and very time-consuming given that the actuating force was generated by an operator attempting to manually open or close the gate valve. Even in valves employing electrical or hydraulic actuators, the relatively large torque required to actuate the valve required that the actuator components be relatively large so that they could readily generate the necessary torque to actuate the gate valve in a timely manner.
However, in recent years, efforts have been made to reduce the torque required to actuate a gate valve. Such reduced torque is desirable because it requires less force to actuate the valve, e.g., the reduction in torque enables the use of smaller, less expensive actuators. Moreover, use of smaller actuators, e.g., electrical or hydraulic motors, is desirable in many applications in which there is limited space availability for such components. Unfortunately, problems may arise when using a relatively low torque actuating assembly, e.g., a roller screw assembly in an unbalanced gate valve. Due to the low friction and operating torque of the roller screw assembly, unbalanced stem forces may be sufficient to “back-drive” the roller screw 30 and thereby raise the gate 16. Of course, such a situation is undesirable from an operational point of view. Thus, when employing reduced force actuating assemblies, such as a roller screw assembly, it is usually necessary to employ a pressure balancing technique or device, e.g., a second pressure balancing stem, in an effort to avoid the “back-drive” situation described above. However, in addition to adding cost and complexity to a valve design, there are many applications where there is limited spaced available. Thus, the use of the pressure balanced stem 22 is undesirable in such a situation.
The present invention is directed to an apparatus and methods for solving, or at least reducing the effects of, some or all of the aforementioned problems.